1. Field of the Invention
The present invention relates to switched mode power converter circuits. More particularly, the invention relates to single-ended, isolated switched mode power supplies that utilize synchronous rectification and post regulation of the secondary side for multiple outputs.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. An isolated forward converter is a particular type of switched mode converter in which an input DC voltage is periodically switched across the primary side of a transformer using one or more power switches, typically provided by MOSFET devices. The transformer provides isolation between the voltage source on the primary side and a load on the secondary side. Two diodes on the secondary side rectify the switched and isolated input voltage, including a forward diode connected in series with secondary winding that conducts current to the load when the input voltage is present across the secondary winding and a free-wheeling diode connected in shunt with the secondary winding that conducts current to the load when the input voltage is not present across the secondary winding.
In order to improve the efficiency of the forward converter circuits, it is known to replace the rectifying diodes with power switches (e.g., MOSFET devices) that are controlled by a pulse width modulator (PWM) or other type of controller. The operation of the power switches may be controlled so that they are turned on and off in synchronism with the switched input voltage. The control signals applied to the power switches must be synchronized as closely as possible to the current inflection points of the output inductor. This synchronous rectification provides higher efficiency over the foregoing forward converter configuration since the forward voltage drop of the power switches is much lower than that of diodes.
A switched mode power converter may include plural secondary windings coupled to the transformer with associated rectification circuits in order to provide plural output voltages. Since output regulation within tight tolerances is more difficult to achieve with multiple output voltages, various post regulation methods have been employed on the secondary side of the power converter to achieve desired output regulation. In one such secondary side post regulation method, a saturable magnetic inductor is coupled in series with the forward diode. This method provides high efficiency and good output regulation, but tends to degrade at light load or no load conditions. Also, overcurrent protection is difficult to implement with this method. Moreover, the saturable inductor is bulky and inefficient at high frequencies. In another secondary side post regulation method, a switching device (e.g., MOSFET) is coupled in series with the forward diode. The switching device may be operated using leading edge (i.e., delayed turn-on) or trailing edge (i.e., advanced turn-off) modulation to regulate the amount of current delivered to the load. This method provides excellent regulation, high efficiency, high frequency operation, and lossless overcurrent protection.
Notwithstanding the advantages of these techniques for improving efficiency and regulation, a switched mode power converter that includes both synchronous rectification and secondary side post regulation has not been heretofore available. Accordingly, it would be desirable to provide a switched mode power converter that combines the advantages of synchronous rectification and post regulation to achieve both high efficiency and tight output voltage regulation. It would be further desirable to provide a multiple output switched mode power converter that includes both synchronous rectification and post regulation.
The present invention provides a switched mode power converter that includes both synchronous rectification and secondary side post regulation.
More particularly, the switched mode power converter comprises a transformer having a primary winding and one or more secondary windings, a primary side power switch coupled to the primary winding and adapted to periodically apply an input voltage to the primary winding, and an output filter operatively coupled to the secondary winding to provide an output voltage and output current. A bidirectional switch comprised of first and second active switch devices is operatively coupled between the secondary winding and the output filter, and a third active switch device is operatively coupled in shunt with the secondary winding and the output filter. The first and second active switches are arranged such that in an inactivated state each one blocks current between the secondary winding and the output filter in an opposite direction. A control circuit is coupled to the first, second and third active switches. The control circuit activates the first and second active switches synchronously with a first voltage state of the primary and secondary windings and activates the third active switch synchronously with a second voltage state of the primary and secondary windings. The control circuit modulates an activation time of the first and second active switches to regulate at least one of the output voltage and the output current.
In an embodiment of the invention, the first and second active switches are coupled together in series between the secondary winding and the output filter. In another embodiment of the invention, the first active switch is coupled to a first end of the secondary winding and the second active switch is coupled to a second end of the secondary winding. The control circuit may be adapted to modulate either a leading edge or a trailing edge of the activation time of the first and second active switches. The first, second and third active switches further comprise MOSFET devices.
In yet another embodiment of the invention, a multiple output power converter is provided in which a plurality of secondary side circuits is coupled to the transformer. Each secondary side circuit provides both synchronous rectification and secondary side post regulation in accordance with the foregoing embodiments to regulate a respective output voltage and/or output current.
In still another embodiment of the invention, a method for regulating performance of a power converter is provided. The method includes the steps of activating first and second switches to couple current from the secondary winding to the output filter synchronously with a first voltage state of the primary and secondary windings, activating a third switch to maintain current flowing through the output filter while bypassing the secondary winding synchronously with a second voltage state of the primary and secondary windings, and modulating activation time of the first and second switches to regulate at least one of the output voltage and the output current. The modulating step may further comprise modulating either a leading edge or a trailing edge of the activation time of the first and second switches.
A more complete understanding of the switched mode power converter having synchronous rectification and secondary side post regulation will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.